H-1 pulsed ENDOR and ESEEM evidence that the bis-imidazole complexes of iron(III) tetraphenylchlorin and tetraphenylporphyrin have the same order of g values, and the same electronic ground state

The electronic structures of the bis-imidazole complexes of iron(III) tetra phenylporphyrin ([(TPP)-Fe(ImH)(2)](+)) and iron(III) tetraphenylchlorin ([ (TPC)Fe(ImH)(2)](+)) in frozen glassy solutions have been studied by the pu lsed electron nuclear double resonance (ENDOR) technique of Mims and by ele ctron spin-echo envelope modulation (ESEEM) spectroscopy. ESEEM spectra hav e been used to determine the orientation of the imidazole ligand planes wit h repect to the g tensor axes. In the ENDOR spectra, the manifestations of the implicit TRIPLE effect described and explained earlier by Doom et al. ( J. Am. Chern. Sec. 1996, 118, 7014) were seen. In this work, the explicit e xpressions describing this effect were derived for the first time and used to successfully simulate the proton ENDOR spectra at the low- (LF) and high -field (HF) edges of the EPR spectrum. Using pulsed ENDOR, we have been abl e to determine the spin density distributions in the pi -systems of both te trapyrroles and show that [(TPC)Fe(ImH)(2)](+) has the electronic orbital g round state (d(xy))(2)(d(xz),d(yz))(3), the same as that known for [(TPP)Fe (ImH)(2)](+), and the largest principal g value corresponds to the g tensor axis 3, which is normal to the heme plane. For the TPP complex, the g tens or axis 1, corresponding to the smallest principal g value, was found to be at an angle phi (1) of 30-35 degrees from the N-Fe-N axis, with the ligand planes rotated by the angle of 20-25 degrees in the opposite direction. Fo r the TPC complex, phi (1) was found to be about 25 degrees from the direct ion N-I-Fe-N-III, where N-I corresponds to the nitrogen of the saturated py rrole ring. The ligand planes in this complex were found to be oriented at an angle of about 10 degrees in the opposite direction.